The present invention relates to a carbonaceous material suitable as an electrode material for a non-aqueous secondary battery, and a process for production thereof. The present invention also relates to an electrode structure comprising such a carbonaceous electrode material, and a non-aqueous secondary battery having such an electrode structure.
Along with size-reduction of electronic devices and apparatus, development of batteries with higher energy densities has been demanded and, in response to such demands, various non-aqueous electrolytic solution batteries, including so-called lithium batteries, have been proposed.
However, a battery using lithium metal for a negative electrode, particularly when constituted as a secondary battery, is accompanied with difficulties, such as (1) inferior charging characteristic, as represented by an ordinary charging time of 5-10 hours, (2) a short cycle life, etc. These difficulties may be all attributable to lithium metal per se and have been assumed to be caused by, e.g., change in form of lithium, formation of lithium dendrite or irreversible change of lithium accompanying the repetition of charge-discharge cycles.
As a measure for obviating the above-mentioned difficulties, it has been proposed to use a carbonaceous material for a negative electrode (e.g., Japanese Laid-Open Patent Application (JP-A) 62-90863, and JP-A 62-122066). This negative electrode has been developed by utilizing a phenomenon that a lithium-intercalated carbon compound is easily formed electrochemically. For example, when a negative electrode comprising carbon is charged in a non-aqueous electrolytic solution, lithium in a lithium-containing positive electrode is electrochemically intercalated between layers of the negative electrode carbon (i.e., the negative electrode carbon is electrochemically doped with lithium). The lithium-doped carbon thus formed functions as a lithium electrode and, during discharge of the battery, the lithium is released (or de-doped) from between the carbon layers to be returned to the positive electrode.
The current or electric capacity per unit weight of a carbonaceous material constituting such a carbonaceous negative electrode (Ah/kg) is determined by the lithium-doping capacity thereof, so that a carbonaceous negative electrode material may desirably show as large a lithium-doping capacity as possible (while a theoretical upper limit is given as one lithium atom per 6 carbon atoms).
Moreover, in order to ensure a sufficient operation time of a compact electronic device or apparatus and a sufficient life of an electricity supply package therefor, a secondary battery as described above is being desired to exhibit a still higher energy density and it is required to develop a carbonaceous electrode material having a still higher lithium doping and de-doping capacities.
In the course of research and development of high-performance carbonaceous electrode materials more suitably used for non-aqueous secondary batteries in view of the above-mentioned problems, we have examined, as a new carbon source, organic materials originated from plant and characterized by plant fiber. As a result, it has been found that a carbonaceous material obtained by appropriately calcining and carbonizing a certain class of fibrous plant materials provides a carbonaceous material having unexpectedly large doping and de-doping capacities, and the carbonaceous material is rich in fine pores suitable for doping with a battery (or cell) active substance, such as lithium, unlike the conventional carbonaceous electrode materials, whereby the present invention has been arrived at.
Accordingly, a principal object of the present invention is to provide a carbonaceous material having large doping and de-doping capacities.
Another object of the present invention is to provide a process for producing such a carbonaceous material, and a non-aqueous secondary battery having an electrode composed of such a carbonaceous material.
According to the present invention, there is provided a carbonaceous electrode material for a non-aqueous secondary battery, comprising a carbonaceous material having a true density as measured by a butanol substitution method of at most 1.46 g/cm3, a true density as measured by a helium substitution method of at least 1.7 g/cm3, a hydrogen-to-carbon atomic ratio H/C of at most 0.15 as measured according to elementary analysis, a BET specific surface area of at most 50 m2/g as measured by nitrogen adsorption BET method, and a carbon dioxide adsorption capacity of at least 10 ml/g.
The carbonaceous material according to the present invention is a so-called non-graphitizable carbon and can store a cell active substance between carbon layers and in fine pores therein. Moreover, the carbonaceous material of the present invention is characterized by a true density as measured by butanol substitution method of at most 1.46 g/cm3 and a true density as measured by helium substitution method of at least 1.70 g/cm3, representing the presence a substantial proportion of fine pores into which butanol is not penetrable but helium can intrude. Such fine pores not penetrable by butanol but intrudable by helium are useful for storing of (i.e., doping with) the active substance and accordingly provide larger doping and de-doping capacities for the active substance by the contribution thereof.
The carbonaceous material according to the present invention may preferably have a potassium content of at most 0.5 wt. % as measured by fluorescent X-ray analysis.
According to the present invention, there is further provided a process for producing a carbonaceous electrode material as described above, comprising: carbonizing an organic material originated from bamboo genera of family Gramineae at a temperature of 1000-1400xc2x0 C. under a reduced pressure or in an inert gas atmosphere.
According to the present invention, there is also provided an electrode structure for a non-aqueous solvent secondary battery, comprising: an electroconductive substrate and a composite electrode layer disposed on at least one surface of the electroconductive substrate; the composite electrode layer comprising the above-mentioned carbonaceous material in a particulate form, and a binder.
According to the present invention, there is further provided a non-aqueous solvent secondary battery, comprising a positive electrode, a negative electrode, and a separator and a non-aqueous electrolytic solution disposed between the positive and negative electrodes; wherein at least one of the positive and negative electrodes comprises an electrode structure as described above.